This invention generally relates to capacity control apparatus for compressors, and more specifically to apparatus for modulating the capacity of a reciprocating compressor.
Considerable attention has been and continues to be directed toward designing heat pump systems which effectively satisfy a relatively large heating load at one time and, at a different time, efficiently meet a comparatively small cooling load. Such heat pump systems would be particularly well suited, for example, for use in the northern regions of the United States. One basic design approach is to provide a heat pump system with a single compressor having a capacity large enough to satisfy the common, large loads placed on the compressor when the heat pump system is used to heat a room or area. Such a compressor, however, has a capacity which is much larger than what is needed to satisfy the usually low loads placed on the compressor when the heat pump system is employed to cool the room or area served thereby, and preferably the compressor operation is controlled or governed in some manner to inhibit overcooling of this room or area and to reduce the associated wasted energy and unnecessary operating costs.
For example, the compressor may simply be cycled on and off as the room or area temperature fluctuates above and below a preset value. This particular arrangement, however, may result in frequent cycling of the compressor into and out of operation, and as is understood in the art, various undesirable characteristics such as reduced lubrication and excessive noise may occur if a compressor is cycled on and off too frequently. For these, and other reasons, simply cycling the compressor on and off to satisfy a low cooling load is often not favored. Alternatively, the compressor may be provided with capacity control apparatus for varying the mass of compressed vapor discharged from the compressor, allowing the compressor to operate continuously for longer periods of time even though only a relatively low load is placed on the compressor. Prior art compressor capacity control apparatus, though, are not as well suited as might be desired for use with compressors which are used in the heat pump applications described above. For example, many compressor capacity control devices reduce the compressor capacity through discrete steps. While this may improve compressor efficiency, in many circumstances the compressor will still cycle on and off much more frequently than is preferred. Further, many prior art compressor capacity control apparatus are expensive to manufacture and are generally complex, and thus may be difficult to install, calibrate, and operate accurately.